The invention concerns a process for cathodic electrodeposition coating (CEC) using water-insoluble organic nitrites and/or nitrates as additives for the cathodic electrodeposition coating compositions in order to improve the adhesion of subsequent coats to coatings produced from them.
Cathodic electrodeposition lacquers (CEC lacquers) are used in particular for producing anti-corrosive primers on metallic substrates, such as car bodies for example. The coating films deposited from electrodeposition lacquers by electrodeposition are then stoved in large circulating air ovens, which can be heated directly or indirectly, and provided with further coating films. Directly heated circulating air ovens are heated by combustion of natural gas, for example, whereby the waste gases are introduced into the circulating air of the oven. In circulating air ovens heated indirectly by means of heat exchangers, no waste gases are introduced into the circulating air of the oven.
It has been established in industrial practice that the adhesion of subsequent coating films applied to stoved cathodic electrodeposition coating films, especially underbody sealant films, is often inadequate if the cathodic electrodeposition primer has been stoved in indirectly heated circulating air ovens.
The object of the invention is the improvement or provision of an adequate adhesion of coating films to cathodic electrodeposition coating films that have previously been stoved in indirectly heated circulating air ovens. This applies particularly in connection with underbody sealant films for application onto cathodic electrodeposition coating films.
The object can be achieved if cathodic electrodeposition lacquers containing scarcely volatile or non-volatile, water-insoluble organic nitrites and/or nitrates are used for cathodic electrodeposition coating films that are to be stoved in indirectly heated circulating air ovens following electrodeposition.
Although JP 52127930 describes electrodeposition coating compositions that can contain various cellulose derivatives, such as e.g. cellulose nitrate, no reference is made to an improvement of the adhesion of subsequent coats to coatings obtained from electrodeposition coating compositions containing organic nitrite and/or nitrate.
DE 36 14 599 C describes a process for stoving cathodic electrodeposition lacquers containing flow-promoting additives in an indirectly heated oven, whereby fillers or underbody sealants for example are applied to the stoved electrodeposition lacquer. A content of nitrogen oxides of at least 1 mg/m3 is established in the oven. This minimum content of nitrogen oxides is necessary to improve the adhesion between the cathodic electrodeposition film and the other coats.